1. Field of the Invention
The present invention relates to a ceramic-metal assembly and a ceramic heater. More specifically, the invention relates to a ceramic heater including an electrode pad of a ceramic-metal assembly having increased adhesion to its ceramic base and a connection terminal.
2. Description of the Related Art
Conventionally, ceramic heaters in which a heating resistor made of a refractory metal such as tungsten or molybdenum is buried in a ceramic base made of alumina or the like are widely used. For example, a ceramic heater that is inserted in a sensor element of a gas sensor is formed by winding a ceramic green sheet incorporating a heating resistor around a ceramic porcelain tube and firing the resulting assembly to form an integral body. The ceramic beater, on its outer circumferential surface, also has electrode pads that are electrically connected to the heating resistor. Also, connection terminals for applying an external voltage to the heating resistor are brazed to the respective electrode pads. Like the heating resistor, the electrode pads are made of a refractory metal such as tungsten or molybdenum.
However, since the ceramic base and the electrode pads are made of different materials, a problem relating to adhesion therebetween may occur. In particular, ceramic heaters tend to be repeatedly used at high temperature or in such a manner as to receive a mechanical load. Therefore, one or both of the ceramic pads may peel off of the ceramic base.
In view of the above, a method has been proposed in which a glass component is introduced into unfired electrode pads from an unfired ceramic base during firing so as to increase the strength of joining of a ceramic base and electrode pads by the bonding ability of the glass component (JP-A-49-076711 and JP-A-57-082188). The unfired ceramic base and the unfired electrode pads become a ceramic base and electrode pads when the firing has been completed. Another method has been reported in which the strength of joining of a ceramic base and electrode pads is increased by applying a joining material containing a ceramic material powder of an unfired ceramic base and a metal powder of unfired electrode pads to the joining surfaces of the unfired ceramic base and the unfired electrode pads followed by firing (JP-A-58-120579).
Ceramic heaters also tend to be used repeatedly at a high temperature or in such manner as to receive a mechanical load. Thus, as is the case for joining the connection terminals and electrode pads, one or both of the connection terminals may peel off of the electrode pads. In this regard, a method for increasing the bonding strength between the electrode pads and the connection terminals is known in which the composition of a brazing material is determined so as to attain a high bonding ability between the electrode pads and the connection terminals (JP-A-11-292649).
3. Problems to be Solved by the Invention
However, particularly in recent methods of using ceramic heaters in which the operating temperature is set at even higher temperatures and having a high repetition frequency or cycling rate of heating and cooling operations, the above-described conventional techniques cannot provide sufficient bonding strength between the ceramic base and the electrode pads and between the electrode pads and the connection terminals. As a result, it has become difficult to obtain ceramic heaters having a sufficiently long life.